Pilot actuated valve systems are generally known in the art and can be utilized in a wide variety of applications. In some applications, pilot valves are utilized to control a pilot fluid that is used to actuate a pressure-actuated main control valve. Pressure-actuated valves typically comprise a biasing piston or other element that actuates the valve when acted upon by a pressurized fluid supply. Generally, pilot valves control a pilot fluid that is at a pressure much less than a pressure of the operating process fluid controlled by the main control valve. By utilizing a pilot fluid with a lower pressure than a pressure of the process fluid being controlled by the main control valve, less force is required to actuate the pilot valve. This is especially true in situations where the pilot valve comprises a solenoid-actuated pilot valve. In some circumstances, the power supplied to the solenoid-actuated pilot valve may be limited in order to reduce power consumption and thus, costs, or because of certain power regulations prescribed by a government or other regulatory agency. The pilot fluid may comprise a pneumatic fluid, a hydraulic fluid, etc. The particular fluid used as the pilot fluid may depend on the particular application.
One particular use of pilot-actuated valve systems is in the control of process gas for blow molding systems. Blow molding is a generally known process for molding a preform part into a desired product. The preform is in the general shape of a tube with an opening at one end for the introduction of pressurized gas, typically air; however, other gases may be used. One specific type of blow molding is stretch blow molding (SBM). In SBM applications, a valve block provides both low and high-pressure gas to expand the preform into a mold cavity. The mold cavity comprises the outer shape of the desired product. SBM can be used in a wide variety of applications; however, one of the most widely used applications is in the production of Polyethylene terephthalate (PET) products, such as drinking bottles. Typically, the SBM process uses a low-pressure fluid supply along with a stretch rod that is inserted into the preform to stretch the preform in a longitudinal direction and radially outward and then uses a high-pressure fluid supply to expand the preform into the mold cavity. Each of the low-pressure and high-pressure fluid supplies can be controlled using a multiple-stage valve system. The resulting product is generally hollow with an exterior shape conforming to the shape of the mold cavity. The gas in the preform is then exhausted through one or more exhaust valves. This process is repeated during each blow molding cycle.
As can be appreciated, with the high speed of the molding cycle that is currently achievable, even small losses in energy during each molding cycle can result in substantial increases in operating costs. As a result, the valve system typically uses a multiple-stage valve system that includes a two-stage pilot valve system to control a main control valve. The two-stage pilot system is generally preferable over a single-stage pilot system in order to minimize the power required by the solenoid valve. One example of a prior art multiple-stage valve system is shown in FIG. 1.
FIG. 1 shows a schematic of a prior art multiple-stage valve system 100. As can be appreciated many of the details of the valve system 100 are omitted in order to simplify the schematic. As shown, the multiple-stage valve system 100 comprises a two-stage pilot system to control a main control valve 103. The multiple-stage valve system 100 comprises a first pilot valve 101, a second pilot valve 102, and the main control valve 103. While the first and second pilot valves comprise 3/2 (three ports, two positions) valves and the main control valve 103 comprises a 2/2 valve, other configurations are known. As can be appreciated, the main control valve 103 can be actuated as described below in order to control a process fluid supply. When the two-stage pilot valve system 100 is used in SBM applications, the main control valve 103 can be used to control a process fluid supply to/from the mold cavity (not shown), for example.
According to the prior art valve system 100, the first pilot valve 101 comprises a spring biased, solenoid-actuated 3/2 valve. The first pilot valve 101 includes a solenoid 104, a spring return 105, a supply port 106, a pilot port 107, and an exhaust port 108. According to the prior art system 100, the supply port 106 can communicate with a pilot fluid supply 109 via a conduit 110. The pilot port 107 can communicate with the second pilot valve 102 via a conduit 111. In the schematic shown, when the solenoid 104 is de-actuated, the spring 105 biases the first pilot valve 101 towards a first position. In the first position, the supply port 106 is closed off from the pilot port 107 while the pilot port 107 can communicate with the exhaust port 108. As a result, fluid pressure in the conduit 111 can exhaust through the exhaust port 108.
As can be seen, a conduit 112 is provided, which branches off from the conduit 110 that is in fluid communication with the pilot pressure supply 109. The conduit 112 provides a fluid communication path between the conduit 110 and the supply port 113 of the second pilot valve 102. A conduit 116 is also provided. The conduit 116 branches off from the conduit 112 and provides fluid communication between the conduit 112 and a first biasing piston 117 of the second pilot valve 102. As is generally known in the art, pressure-actuated valves, such as the pressure-actuated valves 102, 103 can include biasing members, such as the biasing piston 117. Fluid pressure can act on the biasing pistons in order to actuate the valve. The cross-section of the biasing piston along with the pressure of the fluid acting on the biasing piston determines the force that actuates the valve.
In the prior art multiple-stage valve system 100 shown in FIG. 1, the second pilot valve 102 comprises a first biasing piston 117 and a second biasing piston 118. As can be seen, pressure acts on the first biasing piston 117 whenever fluid is provided from the pilot pressure supply 109. As a result, when the solenoid 104 is de-actuated, the second pilot valve 102 is actuated to a first position by the fluid acting on the first biasing piston 117 from the conduit 116. When the second pilot valve 102 is in the first position, the supply port 113 is closed off from the pilot port 114 and the pilot port 114 is in fluid communication with the exhaust port 115. Similarly, the main control valve 103 is actuated to a first position by fluid acting on a first biasing piston 125 from a conduit 124 that branches off from a process fluid conduit 123. As shown, the process fluid conduit 123 is in fluid communication with a process fluid supply 122.
Upon actuating the solenoid 104, the first pilot valve 101 is actuated to a second position where the supply port 106 can communicate with the pilot port 107. In the second position, the pressure delivered from the pilot fluid supply 109 can communicate with the second biasing piston 118 of the second pilot valve 102 via conduit 110, the supply port 106, the pilot port 107, and the conduit 111. With the pilot fluid supply 109 in fluid communication with the second biasing piston 118, the second pilot valve 102 can be actuated to a second position. This is typically possible by providing the second biasing piston 118 with a larger cross-sectional area than the first biasing piston 117. As a result, even if flow in the conduit 111 is less than flow in the conduit 116, due to for example, a smaller nominal bore in the first pilot valve 101, the force provided by the pilot fluid acting on the second biasing piston 118 can be larger than the force provided by the pilot fluid acting on the first biasing piston 117. In some prior art systems, the first and second pistons 117, 118 may be coupled via a common spool, for example.
As the second pilot valve 102 is actuated to the second position, the exhaust port 115 is closed off from the pilot port 114 and the supply port 113 is brought into fluid communication with the pilot port 114. With the pilot port 114 in fluid communication with the supply port 113, pilot fluid can act on the main control valve 103 via the conduit 119. According to the prior art valve system 100, the conduit 119 provides a fluid communication path between the pilot port 114 of the second pilot valve 102 and a second biasing piston 126 of the main control valve 103.
As can be seen, the main control valve 103 comprises a fluid actuated 2/2 valve with a first fluid port 120 and a second fluid port 121. The main control valve 103 controls the flow of a pressurized process fluid from the process fluid source 122 via the conduit 123. The main control valve 103 also includes another conduit 124 that branches off from the conduit 123. The conduit 124 provides a fluid communication path between the conduit 123 and the first biasing piston 125 of the main control valve 103. Therefore, the conduit 124 provides pressure to the first biasing piston 125 in order to actuate the main control valve 103 to a first position. As shown, in the first position, the first and second ports 120, 121 are in fluid communication with one another and therefore, process fluid from the process fluid supply 122 can be delivered through the main control valve 103.
As mentioned above, upon actuating the solenoid 104, pressurized pilot fluid is provided through the second pilot valve 102 to a second biasing piston 126 of the main control valve 103. With the pilot fluid acting on the second biasing piston 126, the main control valve 103 is actuated to a second position where the first and second ports 120, 121 are closed off from one another.
A two-stage pilot system, such as used in the multiple-stage valve system 100 may be desired for a number of reasons. One reason for providing a two-stage pilot system is to increase the pressure applied to the main control valve 103 while minimizing the power required to actuate the solenoid 104. For example, the nominal bore size of the first pilot valve 101 may be much smaller than the nominal bore size of the second pilot valve 102. As a result, the flow through the first pilot valve 101 may be restricted thereby decreasing the power required to actuate the first pilot valve 101. This allows a pilot fluid with a higher pressure while minimizing the size of the solenoid 104. The size of the first and second biasing pistons 117, 118 can simply be adjusted in order to allow the force of the pressure acting on the second biasing piston 118 to overcome the force of the pressure acting on the first biasing piston 117. The nominal bore size of the second pilot valve 102 can be increased in order to allow a greater flow rate through the second pilot valve 102 thereby allowing a greater flow to be applied to the second biasing member 126 of the main control valve 103.
While a multiple-stage valve system may be desirable in some circumstances, the prior art multiple-stage valve system 100 has a number of drawbacks. One drawback is that due to a difference in pressure between the process fluid supply 122 and the pilot fluid supply 109, the second biasing piston 126 is required to have a relatively large cross-sectional area compared to the first biasing piston 125 of the main control valve 103. This is because the force provided by the pressurized pilot fluid acting on the second biasing piston 126 is required to be greater than the force provided by the pressurized process fluid acting on the first biasing piston 125. The enlarged cross-section required for the second biasing piston 126 results in a much larger valve block, thereby significantly increasing the cost associated with manufacturing the valve system 100. Further, if there is a change in either the pressure of the process fluid supply or the pilot fluid supply, the ratio of the cross sectional area of the first biasing piston 125 to the second biasing piston 126 may no longer provide the desired forces required to properly and efficiently actuate the valves. Therefore, the prior art two-stage pilot valve system 100 shown in FIG. 1 is susceptible to problems due to variations in pressure.
The present invention overcomes these and other problems and an advance in the art is achieved. The present invention provides a multiple-stage valve system that utilizes the process fluid rather than the pilot fluid to actuate the main control valve between two or more positions. As a result, the cross-sectional areas of the first and second biasing pistons can remain constant over a wide variety of pressures. Further, because substantially the same pressure acts on both the first and second biasing pistons of the main control valve, the cross-sectional area of the second biasing piston can be formed substantially smaller than in the prior art valve system 100. Therefore, the size of the valve block can be significantly reduced. Further, the present invention utilizes the process fluid to bias the second pilot valve in a first direction to a first position. The second pilot valve also controls the flow of the process fluid to the main control valve and not the pilot fluid as in prior art designs.